1. Field of Invention
The invention relates to methods and apparatus for determining background content of an image of a scanned document and using this content to dynamically adjust the black point of the image.
2. Description of Related Art
In conventional scanners and digital copiers, the black point of an image is adjusted statically using a fixed offset value. Such a method works fine for images with white or light background. However, when the input image has a darker background, such as colored paper, the blackest point is also lower and using a fixed offset value causes the background to not be completely suppressed. That is, the use of a single fixed offset for different types of input images (images on various colored backgrounds) does not work very well in suppressing both the background while reproducing quality black text.
One approach to this is to provide several preset modes, each having different offset values. However, picking a different mode with a lower offset value causes the dark areas to not be reproduced as solid black. Another problem with this approach is that a user has to manually set or pick a correct mode. This is inconvenient.
In copier and scanner systems, copying a document or an original while suppressing the substrate of the original is often required. For example, when the original is printed on colored paper. Such suppression can be achieved by detecting the background color automatically. Background detection can be performed on just the leading edge of the document or the whole document. However, whole page background detection requires pre-scanning of the entire original image. The detected background can be removed by adjusting the gain of the scanned image and clipping the values that exceed the system processing range.
Automatic background suppression senses the background and automatically suppresses it before final printing. Conventional automatic background suppression systems generate a histogram of the document using standard methods and then calculate the mean and standard deviation. Other systems estimate the standard deviation and mean value. Exemplary background detection systems and methods can be found in co-pending U.S. patent application Ser. No. 08/886,205 filed Jul. 1, 1997, copending U.S. application Ser. No. 09/159,038 filed Sep. 23, 1998, and U.S. Pat. No. 5,881,166, the disclosures of which are incorporated herein by reference in their entirety.
The above systems and methods have been used to suppress background and to adjust a tone reproduction curve (TRC). However, such systems have not heretofore used such background information to scale the offset value. Applicants have discovered that the background value determined during a scan of the leading edge of the document can be used to dynamically adjust the fixed offset value for the image. This helps in increasing the dynamic range of the output image without sacrificing the black areas of the image. Moreover, as the histogram only acts on the leading edge, the scan and approximations can be achieved in real-time, thus avoiding the delays necessary to scan an entire image to determine precise white point and black point values.
Thus, a method of scanning a document according to the invention may include: scanning at least a portion of a document; automatically detecting a background of the document to form a white point WP of the image; providing a default fixed offset value FOV used as an initial black point BP of the image; providing a scaled adjustment factor AF dependent on the detected white point WP; providing a scaled black point BP by multiplying the fixed offset value FOV by the scaled offset adjustment factor AF; computing a gain G according to the equation G=255/(WPxe2x88x92BP); and scanning the image and applying the gain G and black point BP to all pixels of the image.
Also, a digital scanner or digital copier according to the invention may include: a scanning unit that can scan the document; a histogram generator that generates a histogram from scanned data corresponding to at least a portion of the scanned document; a computing unit; a fixed offset value FOV stored in memory; and an offset adjuster that calculates an offset adjustment factor AF based on a detected white point WP of the document, wherein the computing unit calculates the white point WP based on the histogram, calculates a black point BP based on the fixed offset value FOV and the detected white point WP, and calculates a gain based on the calculated white point WP and black point BP.
These and other features and advantages of this invention are described in or are apparent from the following detailed description of various exemplary embodiments of the systems and methods according to this invention.